Resistor composition and element



United States Patent 3,551,355 RESISTOR COMPOSITION AND ELEMENT Jason D.Provance, Glendora, Califi, assignor to Bourns, Inc., a corporation ofCalifornia No Drawing. Filed July 12, 1967, Ser. No. 652,674 Int. Cl.H01b 1/06; C03c 3/04 US. Cl. 252514 12 Claims ABSTRACT OF THE DISCLOSUREAn electrical resistance composition and element of the cermet type fora potentiometer or variable resistor, characterized by low resistivityand low metal content and low contact-resistance variation, thecomposition comprising as a glassy matrix the product of a glassmakingoperation in which a nucleating agent is included with constituents of alead borosilicate glass, the product thus comprising numerouscrystallites, the composition further comprising within the glassymatrix a dispersion of metal selected from among the platinum family,the composition being adapted to form a film characterized by lowresistivity of the order of 100 ohms per square (sheet resistivity,e.g., a film one inch square and one mil thick) and a low metal contentof the order of less than 7% by weight, and low contact resistancevariation when fired.

Herein for convenience and in the interest of brevity, the term glass isused to denote a product of a glassmaking operation wherein there isincluded with glassmaking constituents a nucleating agent or componentwhereby the product contains numerous crystallites and hence is notamorphous or vitreous and thus in a strict sense is not a true glass.

BACKGROUND OF THE INVENTION In the field of potentiometer resistanceelements not using wire as a resistive material, it is widely known touse an insulative ceramic base to support a wear-resistant glassycomposition film or layer of electrically resistive material and whichfilm or layer is formed from an applied coat of liquid carrier carryingfinely-divided glass comprising conductive metal, the film or layerbeing fired in situ to provide a hard resistive layer firmly bonded tothe insulative base. In all types of potentiometers and variableresistors, it is desirable that the contact resistance (CR), that is,the electrical resistance exhibited between the movable contact and theelement or film, be a minimum and, more importantly, that the contactresistance variation (CRV) be a minimum as the contact is moved alongthe film or element. In general in the prior art, low-resistanceelements could be obtained concurrently characterized by low values ofCRV, only when the metal content of the layer or film was increased toabove about 7% (by weight) of the film to provide the lower resistivitynecessary in the lower-resistance ranges. However, the large amount ofmetal thus introduced into the film invariably caused the surfaces ofsuch lower-resistance range elements to be highly abrasive, withresultant excessive wear of the wiper or contact. The present inventionprovides resistor elements whose resistivities are Within thelower-resistance ranges desired, using low-metal-content material,whereby there are retained the desirable low contact-resistancevariation and non-abrasive surface characteristics of thelow-metalcontent higher-resistivity range film-elements previouslyattainable. This the invention accomplishes by utilizing alow-metal-content mixture of a finely-divided glassy material formed bya glassmaking procedure in which a nucleating agent or agents isincluded with the oxides "ice used in the glassmaking process, andmetal-containing material, the two components being very finely dividedand thoroughly intermixed and applied as a suspended component of aliquid-mix (by such procedures as silkscreening, roller coating,dipping, or the like), followed by subsequent firing to remove organicand other volatile materials and to form the adhering film with adesired matte surface. The glassy material produced by nucleation of theglass forming oxides into a crystallitecontaining glass is no longer aglass in the strict sense, since it contains innumerable crystallites,and the crystallite structure is at least to a considerable extentresponsible for the aforementioned lower resistance obtained with metalcontents of less than about 7% and is retained in the finally producedfilm as a matrix in and on which metal is incorporated.

Prior art procedures that probably most closely approach those used inthe production of the presentlyclaimed resistance elements aredelineated in U.S. patents to Place, Sr., et al., No. 3,149,002, and toHofiman, No. 3,207,706. In the former, use is made of nonconductingmaterial such as glass interspersed with metal to provide the highresistance values that are otherwise attainable only with metal filmsthat are too thin to successfully withstand the wearing effect of thewiper during repetitive operation of the latter. Thus means and methodsare therein disclosed for making satisfactory small-size resistanceelements having total resistance values so high as to be not attainablewith wire-wound elements. However use of the techniques and materialstherein taught, for making elements of low values of total resistance(TR), results in the aforenoted highly abrasive surface which causesexcessive wiper wear and which surface is characterized by excessiveCRV. In the process set out in Pat. No. 3,207,706, glass frit containingaluminum oxide (A1 0 is mixed with metal particles of silver orpalladium to provide a resistor which has a lower value of temperaturecoeflicient of resistivity (TCR) than the previously known vitreousresistance elements. In that process as well, the low values ofresistivity required for resistors of low ohmic value are obtained onlyby increase of the metal content to those high values (in excess ofabout 7%) that result in the noted highly abrasive surface characterizedby undesirably high CRV.

In accord with the present invention, an insulative base such as one ofalumina or the like is used, with metalfilm terminals at the ends toreceive or connect with the resistive film or layer. The film is formedas the product of a fired composition or mixture applied in paste-likeform by one of the previously mentioned procedures such as silkscreening, the applied mixture being one comprising a finely-dividedpowder of the product produced by fritting the product produced bynucleating the components of a lead borosilicate glass, and the other ametallic salt, both dispersed together in a liquid vehicle. The objectof the procedure is to secure glassy films of low resistivity with lowmetal content and characterized by low CRV. By glassy is meant havingappearance and some other characteristics of glass, but not being anamorphous structure. That object is attained by producing by chemicalreduction in situ enough molecules of metal from one of theglass-forming materials (as, for example, by reduction of a fractionalportion of PhD molecules to metallic lead) along edges and/ orinterfaces of the crystallites, whereby a multiplicity of conductivepaths are formed, together with reduction of the added metallic salt toconductive metal along crystallite boundaries and interspersedthroughout the remaining glassy material or crystallites, to providemultiplicities of conductive paths through the length of the film, Thenoted desired result may in most instances be attained by a combinationof the two phenomena, the conductive paths being both through thecrystallites and via the intercrystallite material. Since the percentageof metal content in the improved film is at least as low as that in thesomewhat similar resistance elements of far higher resistivity disclosedin the aforemention d prior art which are characterized by low CRV andnon-abrasive surface, the elements thus made according to the inventionare characterized by low resisitivity, low metal content and low CRV,and are similarly non-abrasive.

The preceding brief general description of the invention makes itevident that it is a principal object of the invention to provideimprovements in resistive compositions for film type resistanceelements.

Another object of the invention is to provide improvements in processesfor producing electrical resistance devices.

Another object of the invention is to provide improvements in themanufacture of glassy crystallite materials.

Another object of the invention is to provide an improved material foruse in the manufacture of resistors.

Another object of the invention is to provide a cermet resistanceelement characterized by a resistive film of very low resistivity andmetal content less than five percent by weight of the film.

Another object of the invention is to provide a cermet resistanceelement characterized by very low film resistivity of less than amaximum value of about 500 ohms per square and low CRV of the order of1% or less.

Other objects and advantages of the invention are set out or madeevident in the appended claims and the following description of apreferred mode and products of the invention.

The frit component of the mixture is prepared by admixing and thoroughlyblending high-purity glassmaking materials, including one suchconstituent as zinc oxide common in the art of glassmaking but which inan alumina, lead, borosilicate glass system reacts in such a manner asto form a devitrified glass, in carefully measured amounts, melting atabout 900 C. the admixed materials in an alumina-silica refractorycrucible, and fritting the homogeneous molten product by pouring it intodistilled water. The frit is thoroughly dried, then ground in amechanical grinder of the mortar and pestle type. Typical mesh sizes ofthe ground nucleated product thus produced, by weight percentages, are:

Retained on 100-mesh screen 0.0 Retained on 250-mesh screen 35.2Retained on 270-mesh screen 9.2 Retained on 325-mesh screen 5.0 Passedby 325-mesh screen 50.6

The thus produced divided glass or product is further reduced as toparticle size in a vibratory mill, using an organic solvent such asmethanol or isopropyl alcohol as a carrier. The reduction is carried tothe point where average particle size is between 2 and 5 microns.

As is made evident by consideration of exemplary data hereinaftertabulated, the frit according to the invention contains, by molepercentage, of the order of from 3% to 21% of SiO from 50% to 66% ofPhD, from 4.5% to 22% of B 0 from 6% to 15.5% of ZnO and from 6% to15.5% of A1 0 but in relative proportions set out in the tabulation, itbeing noted that ZnO is essential to promote crystallization and improvethe TCR (temperature coefiicient of resistivity). In the latter matter,it should be noted that the absence of ZnO from the glass (as indicatedin composition No. V) results in much higher CRV and poor TCR. B 0 isessential to low resistivity in that the solubility and homogenizationof the metal particles is increased. Boric oxide also contributes toimproved resistance to the effects of moisture and elevatedtemperatures. The A1 0 component acts as a stabilizer againstdeterioration of the glass by water and acids, and promotes low TCR. ThePhD component contributes to low total resistance. SiO contributes toimproved moisture resistance, thermal sability and low TCR.

The following tabulation portrays the chemical components orcompositions and the physical properties of. the glass components of theresistive compositions, and also the glass/ metal weight ratio of theresistive composition of the fired film elements, the thermal stabilitycharacteristics, moisture stability characteristics, and CRVs, ofelevent representative compositions labeled I to XI, inclusive. Thetabulation is for reasons of clarity and space requirement divided intotwo sections, namely Table I and Table Ia.

TABLE I I II III IV V VI S102, mole percent. 14. 10. 65 12.27 13. 2814.48 20. 18 PhD, mole percent- 63.18 65.46 52. 76 57.28 62.06 52.66B203, mole percent 4 90 11.17 12. 73 13.76 14.98 12. 75 Z110, molepercent 8. 61 6. 36 15. 03 7. 84 7. 28 1 03 8.61 6.36 7.21 7.84 8.487.13 Glass properties:

I.S.P. C) 347 318 333 346 366 364 Expansion, Tce. 105.0 127. 6 105.1103.0 98. 0 98. 0 Density, g./cc 6. 82 6. 76 6. 36 6.32 6. 17 6.08Resistance material:

Glass, percent 97. 22 97. 19 97.03 97.00 96. 93 96. 89 Metal, percent 2.78 2. 81 2. 97 3.00 3.07 3. 11 Resistor properties:

Resistivity range,

kilo-Ohms Thermal stability,

percent 0.2 -0. 7 1.4 1. 7 0.4 1. 0 Moisture stability,

percent 0.2 -0. 1 0.9 0.08 0.2 0.2 C RV, percent 0.5 1.0 0.5 1. 0 1. 51.0

1 From 1.50 (650 C.) to 7.00 (850 C.) 2 From .25 (650 C.) to 2.20 (8500.). 3 From .13 (700 C.) to 2.8) (500 0.). 4 From .13 (700 C.) to .55(500 0.). 6 From .09 (850 C.) to .25 (650 .C.) 4 From .30 (700 C.) to.65 (500 C.).

TABLE Ia VII VIII IX X XI S102, mole percent 15. 50 12.23 14.47 5. 5612. 1O PbO, mole percent. 50.07 52. 63 62. 01 62. 40 51. 89 B203, molepercent. 16. 10 12. 69 14. 97 15.01 21. 79 ZnO, mole percent..- 9.24 7.28 8. 55 8.60 7. 11 A1203, mole percent 9.09 15. 17 8. 43 7. 11 Glassproperties:

I.S.P. C.) 306 373 366 338 366 Expansion, 'Ice. 95. 2 89. 5 100.0 103. 991. 2 Density, g./cc 5. 97 5. 93 5. 78 6. 34 5. 98 Resistance material:

Glass, percent 96.83 96. 81 96. 73 97.01 90. 84 Metal, percent 3. 17 3.19 3. 27 2. 99 3. 16 Resistor properties: Resistivity range,

kilo-ohms Thermal stability, percent. 0. 7 0.8 0. 8 2. 1 0.3 Moisturestability, pcreent 0 5 0. 9 0.9 0.2 0. 5 CRV, percent 0.5 0.5 1.0 1.01.0

1 From .30 (700 C.) to .55 (500 C.). 2 From .25 (600 C.) to .35 (700C.). 3 From .08 (700 C.) to .40 (500 C.). 4 From .15 (700 C.) to .50(600 0.). 5 From .20 (750 C.) to 1.00 (850 0.).

In the preceding tables, the glass-cOmpOsitiOn data are in terms of molepercentages of the several oxides, and the interferometer softeningpoints of the crystallite-containing glassy materials (glasses) aredenoted I.S.P., and indicate the interferometer softening point on thetemperature scale. The softening point values given are, as indicated,in degrees centigrade. The interferometer softening point is arelatively precisely measurable temperature and thus provides a basisfor accurate comparisons among glassy materials. It is related to theless accurately determinable melting temperature of the material in awell understood relationship. In the tables, the resistance compositionglass-to-metals data are in terms of weight percentages. For example,the weight percentages of glass and metal in Example VII (Table Ia) areglass 96.83% and metal 3.17%. From that set of data it is forciblyevident that the resistive films according to the invention compriseexceptionally high percentages of glass and unusually low percentages ofmetal (the maximum metal percentage among the eleven examples being3.27%

Further in the tables, the resistivity ranges data are in terms ofkilo-ohms per square. It will be noted that the resistivity of a filmmaterial having a given glass to metal ratio (97.22% to 2.78% in thecase of Example I), may be made to fall at a determinable value within afairly wide range of resistivity values (from 1.50 kilo-ohms per squareto 7.00 kilo-ohms per square in the cited Example I), depending upon thetemperature to which the film is heated during the firing operation.Firing temperatures range from about 500 C. to about 850 C., and firingdurations are of the order of ten minutes. As indicated in theresistivity range data section of the tabulations, resistivity valuesmay increase with increase of firing temperature (for example from 1.50kilo-ohms per square with a firing temperature of 650 C. to 7.00kiloohms per square with a firing temperature of 850 C. in the case ofthe material of Example I) or the resistivity of some resistancematerials may decrease with increasing firing temperatures, as indicatedin the data for Example III. Thus resistivity may be regulated, and adesired particular value within the indicated limits attained byappropriate selection of the firing temperature in relation to theexposure time during firing.

In the tabulations, further, the linear-thermal coefficient of expansion(Tee) of the glassy product of the glassmaking operation, is given interms of numbers which must be multiplied in each case by to give theunit expansion per unit of length per degree Centigrade. The data arevalid within the temperature range from 22 C. to 300 C.

In the tabulations the thermal stabilities and moisture stabilities arerecorded in terms of percentage shift of total resistance of the firedfilm after 24 hours at 175 C. in the case of thermal stability, and interms of percentage shift of total resistance after 24 hours in water inthe case of moisture stability.

The contact resistance variation (CRV) of the finished exemplaryresistive films is in each example given in terms of percentage of thetotal resistance variation of the element between the terminations. Itwill be noted from the tabulations that the CRV values range between0.5% and 1.5%, in contrast to the usual CRV values of the order of from5% to in the case of cermet resistive films of comparably lowresistivity values.

The resistive compositions or mixes are made as previously indicated,those of Examples III, V and VIII being typical; additional examples areshown following, the quantities shown being by weight in any convenientunit:

Following preparation of the micronized frit, a resistor composition ormix is prepared by mixing thoroughly frit and a salt of the metal to beincorporated in the final resistive film (for example, Rh C1 Ru C1 inthe weight proportions approximately 80% frit and metal salt andvehicle, the salt being a dispersion in triethylene glycol or othersimilar liquid as a vehicle, in a ratio of about 1:2. A suflicientamount of the vehicle liquid is added to bring the solids to liquidratio to 2:1 by weight, for example, whereby to facilitate spreading,silk screening, or other procedure used in forming a film of thesuspension or mix on a base of alumina or other suitable fired ceramic.As will be evident to those skilled in the art, the vehicle may beglycerol, octyl alcohol, mineral thinner, ethylene glycol, triethyleneglycol, or combinations of two or more such known carriers. The liquid,the metal salt, and the frit are thoroughly mixed together to form apaste for application to the base.

6 Resistance film mixtures are made up as indicated in the following,those of Examples III, V and VIII of the previous table being cited asexemplary:

All of the exemplary values are in arbitrary weight units, e.g., grams,ounces, pounds, etc.

Consideration of the data comprised in Tables I and Ia clearly indicatesthat the glassy component of the mixture is characterized in eachinstance by a low softening temperature, e.g., 318 C. for Example II to366 C. for several others. The advantages of the low softening pointsare known to those skilled in the art, it being evident that manufactureof cermet resistance elements requiring low fusing or maturitytemperatures to attain a particular resistance can be effected with lessexpensive apparatus, less expenditure of time and heat energy, and lesscorrosive deterioration of the apparatus. The glassy materials fromwhich the resistance compositions are derived are particularly adaptableto manufacturing methods employing in expensive electric or gas-firedfurnaces; the melting and refining temperatures (less than 900 C.) ofthe glassy compositions are well below those temperatures required forconventional or more common types of glasses, or substrate. Theviscosity of the thus prepared mix or paste may be adjusted by variationof the amount of liquid added, or by otherwise changing the liquid tosolids ratio in order to facilitate application by one method oranother.

The prepared mix or paste is applied to the substrate by any suitablemeans and process, as by a silk screen apparatus and procedure to form alayer or film on the base. The film is then dried at a temperature ofthe order of 175 C., either separately or in a continuous firingoperation, whereby the organic material is driven out or removed.Thereafter the base or substrate with the applied film is subjected ineither a separate or continuous firing operation to a temperature offrom 500 C. to 850 C., as indicated in the tables, for a period of theorder of ten minutes. The metal particles remaining from decompositionof the metal salt remain dispersed in and/or on the particles orcrystallities and throughout the glassy matrix of the devitrified glass.The exact submicroscopic details of the structure are unknown but aresuch that with a very low content of metal, very low final filmresistivity is attained (e.g., ohms per square) while retaining the verylow CRV previously associated only with much higher metal content andwithout the aforementioned abrasive surfaces caused by the high metalcontent. Further, the structure is such that TCR values areexceptionally low, being in the range of from 80 parts per million toparts per million. Thus Where in the prior art resistance elementshaving a sufliciently high metal content to produce low resistivitiesthe CRV was of the order of from 5% to 15% of total element resistance(TR), elements made as outlined above are characterized by CRV values ofthe order of only 0.5% to 1.0% of TR.

In those instances wherein the TCR is deemed to be higher thandesirable, a trade-off of a slight increase in resistivity for arelatively large reduction in TCR may be etfected by substituting for aportion of the ruthenium a small percentage (1%10%) of Rhodium, thesubstitution being by a decrease of Ru salt and corresponding additionof Rh salt.

The glasses in Tables I and Ia are derived from combinations of oxidesand compounds as indicated, that of Example VIII being exemplary. Thatcombination (Example VIII) is obtained as indicated by the following:

The preceding disclosure of the principles of the invcntion and a rangeof typical preferred compositions all within the specific generalcomposition and all having the same general novel properties, indicatesfull attainment of the aforementioned objectives. As is made evident bythe examples covering the range of the invention, changes within theboundaries of the invention and within the true spirit and scope of theinvention will occur to those skilled in the art and hence it is notdesired to restrict the invention to exact proportions specified in theseveral examples other than as is required by the appended claims.

I claim:

1. A resistive film for a potentiometer or like variable resistor havinga contact arranged to brush on the film,

said resistive film consisting essentially of the fused mixture of acrystallite-containing frit and interspersed conductive particles ofwhich particles the major part is ruthenium, the said frit comprisingthe fused product of glassmaking oxides and a nucleating agent, theglassmaking oxides including PbO, SiO and B and the said nucleatingagent including ZnO, the percentage of said conductive particles in saidresistive film being in the range of from 1% to 7% by weight,

said resistive film further being characterized by contact-resistancevariation not in excess of 1.5% of the total resistance of saidresistive film between the terminals thereof, and further beingcharacterized by a matte-type non-abrasive surface and by thinfilmelectrical resistivity within the range from 80 ohms/ square and 7000ohms/square.

2. A resistive film according to claim 1, in which said film comprisesruthenium in the range from 2.78% to 3.2% by weight of said film and thethin-film resistivity of said film is of the order of 80 ohms/square.

3. A resistive film according to claim 1, in which said film comprisesamong said conductive particles rhodium to the extent of from about 0.1%to about 10% of the total metal in the film, by weight.

4. A resistive film according to claim 1, in which the mole percentageof the glassmaking oxides and nucleating agent are within the ranges offrom 50% to 66% PbO, from 4.5% to 22% of B 0 and from 6% to 15.5% ZnO,

from 3% to 21% of SiO and from 6% to 15.5% A1 0 respectively.

5. A resistive film according to claim 1, in which the mole percentagesof said glassmaking oxides and nucleating agent are: PbO, 63.18%; SiO14.70%; B 0 4.90%; and ZnO, 8.61% and A1 0 8.6 1%, respectively.

6. A resistive film according to claim 1, in which the mole percentagesof said glassmaking oxides and nucleating agent are: PbO, 51.89%; SiO12.10%; B 0 21.79%; and ZnO, 7.1 1% and A1 0 7.11%, respectively.

7. A resistive film according to claim 1, in which the mole percentagesof said glassmaking oxides and nucleating agent are: PbO, 62.01%; SiO14.47%; B 0 14.97%, and ZnO, 8.55%, respectively.

8. A resistive film according to claim 1, in which the mole percentagesof said glassmaking oxides and nucleating agent are: PbO, 52.76%; SiO12.27%;B O 12.73%; and ZnO, 15.03%; and A1 0 7.21%, respectively.

9. A resistive film according to claim 1, in which the mole percentagesof said glassmaking oxides and nucleating agent are: PbO, 52.63; S1012.23%; B 0 12.69%; and ZnO, 7.28%; and A1 0 15.17%, respectively.

10. A resistive film according to claim 1, in which the mole percentagesof said glassmaking oxides and nucleating agent are: PbO, 62.40%; SiO5.56%; B 0 15.01; and ZnO, 8.60%; and A1 0 8.43%, respectively.

11. A resistive film according to claim 1, in which the mole percentagesof said glassmaking oxides and nucleating agent are: PbO, 65.46%; SiO10.65%; B 0 11.17%; and ZnO, 6.36%; and A1 0 6.36%, respectively.

12. A resistive film according to claim 1, in which the mole percentagesof said glassmaking oxides and nucleating agent are: PbO, 57.28%;SiO13.28%; B 0 13.76%; and ZnO, 7.84%; and A1 0 7.84%, respectively.

References Cited UNITED STATES PATENTS 2,739,901 3/1956 Herold et al.10653 3,425,817 2/1969 Ikeda et al. 10653 3,441,422 4/1969 Graft 106-533,462,252 8/1969 Veres 106-53 2,924,540 2/1960' DAndrea 2525l4 3,149,0029/1964 Place 252-5 14 3,154,503 10/1964 Janakirama-Rao 2525l4 3,207,7069/1965 Hoffman 2525l4 3,252,831 5/1966 Ragan 2525l4 3,271,193 9/1966Boykin 2525l4 3,329,526 4/1967 Daily et a1. 2525l4 3,352,797 11/1967 Kim2525l4 DOUGLAS J. DRUMMOND, Primary Examiner US. Cl. X.R. 252518; 10653UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No..3,551,355 December 29, 197

Jason D. Provance It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected asshown below:

Column 2, lines 29 and 30, "However" should read How "i Column 4, line10, "elevent" should read eleven Column 5, line 75, "base." should readbase Column 6, 31, glasses," should read glasses. Column 5, line 45beginning with The resistive compositions" cancel all to am including"for application to the base" in line 75, same c011 5, and insert thesame after common types of glasses." in

column 6, line 31. Column 6, line 28, "in expensive" should 1inexpensive Column 8, line 19, "PbO, 52.63" should re: PbO, 52.63% 3lines 25 and 26, "B 0 15.01;" should re:

Signed and sealed this 25th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLEI 'Attesting OfficerCommissioner of Pa-

